This invention relates to gun tubes and more particularly to light weight, high strength, high stiffness composite gun tubes having prestressed liners exhibiting greatly improved resistance to the erosive effects of high temperature propellant combustion products.
Gun tubes are pressure vessels that must be made of high strength materials in order to reliably contain very high internal propellant gas pressures, often under adverse conditions simultaneously imposed by the extremely hot and highly corrosive nature of the gaseous combustion products present. In order to hold gun tube wall thickness within reasonable limits so that maximum system weight requirements are not exceeded, various construction techniques have been developed to impart a residual state of compressive stress to the interior portion of the tube. Such compressive stresses cancel in part the large tensile stresses induced by the propellant gas pressure upon firing and thereby reduce the total material wall thickness required for the tube to withstand a given maximum firing pressure.
Three common techniques currently or formerly employed for prestressing gun tubes during fabrication are (1) wire-wrapping under tension, (2) shrink fit assembly of two or more concentric tubes, and (3) autofrettage or self-hooping. The first of these involves the radial wrapping of a central tube or liner with high tensile strength wire, usually square in cross section, while maintaining a uniform tension on the wire and thereby inducing an initial compressive stress in the tube. However, the tendency of wire-wrapped composite tubes to droop and whip excessively has resulted in the general replacement of this type of construction by the shrink-fit assembly method.
The shrink-fit method of composite gun tube construction involves the thermal expansion of one cylindrical tube by heating, the insertion of a second tube concentrically within the first, and then the achievement of a precisely determined interference fit when the heated cylinder is allowed to cool and shrink back toward its original diameter. This procedure results in a jacketed or layered gun tube consisting of two or more concentric cylinders, the total wall thickness of which is less than a non-prestressed monobloc tube of the same strength.
The difficulty and limitation inherent in the method of shrink-fit construction derives from the necessarily small clearance between the heated tube and the cool liner which can be obtained via thermal expansion to produce a given interference fit. During the assembly process this clearance tends to disappear as the liner is inserted because the heat transfer between the two components is usually rapid. Therefore the available insertion time is correspondingly short and long or massive tubes are difficult to assemble.
Autofrettage or self-hooping is a method for prestressing thickwalled steel cylinders which is now the primary method of gun tube manufacture. It involves the technique of stressing the inner portion of the wall beyond the elastic region while at the same time not exceeding the yield point on the outer surface of the tube. When the stress inducing agent, either hydrostatic pressure or an oversized mandrel, is removed, the outer portion of the tube will contract in an effort to recover its original size. The contraction compresses the inner wall of the tube and induces substantial permanent compressive stresses in this region. Prestressed in this manner the tube is safe for any pressure which does not exceed the autofrettage pressure or its equivalent, but the method is generally limited to the manufacture of thick wall steel monobloc gun tubes.